I. Field of the Invention
This invention relates to drilling motors for downhole applications and, in particular, to a composite stator construction for the drilling motor which improves the pumping capabilities of the motor by providing an elastomer coating over a rigid stator former. Alternatively, the elastomeric coating may be applied to the rotor.
II. Description of the Prior Art
Downhole drilling motors provide direct bit drive in directional drilling or deep drilling by pumping drilling fluid through the motor. The working portion of the motor comprises an outer casing having a multi-lobed stator mounted therein and a multi-lobed rotor disposed within the stator. Typically, the rotor has one less lobe than the stator to facilitate pumping rotation. The rotor and stator interengage at surfaces shaped in the form of helical lobes to form a sealing surface which is acted upon by the drilling fluid to drive the rotor within the stator. In the prior systems, one or the other of the stator/rotor is made of an elastomeric material to maintain a seal therebetween.
In the present design of stators, the elastomer is continuous from the interior helical surface to the outer cylindrical surface which is bonded to the outer casing of the motor. Because of variations in the thickness of the elastomer material of the prior known stators, selection of the elastomer's physical properties necessitates a compromise between a high modulus value to preserve the shape of the lobes under operating stresses and the need to affect a satisfactory seal between the inner surface of the stator and the outer surface of the rotor. As the rotor rotates and precesses within the stator, a seal is formed at each point of contact. However, it is difficult to produce satisfactory elastomer moldings which are rigid enough to prevent distortion of the stator surface. In the event the bit torque exceeds the hydraulic torque developed by the motor while the drill string is rotated, the stator will overrun the rotor damaging the elastomer. Furthermore, a variable thickness elastomer generates heat in the core which leads to premature deterioration in the material properties.